Referring to FIGS. 1 and 2, a conventional scanner includes a lid 10, a scanner body 20 and a printed circuit board (PCB) 30. The lid 10 is for flattening a scanned document 70. The scanner body 20 houses the main mechanisms of the scanner, including a transmission mechanism 21, an optical module 22 and a guiding track 23. The transmission mechanism 21 drives the optical module 22 moving reciprocally along the guiding track 23. The optical module 22 includes a light source 221, a reflecting mirror set 222 and a charge-coupled device (CCD) 223. The light source 221 emits light and projects the light to the scanned document 70. The scanned document 70 reflects the light to the reflecting mirror set 222, which reflects the light to the CCD 223. The PCB 30 receives a driving signal and drives the transmission mechanism 21, to move the optical module 22 reciprocally along the guiding track 23 in the scanner body 20. Meanwhile, the optical module 22 reads the scanned document 70 during the scanning movement to obtain image signals. The image signals are transformed to digital signals, which are transferred to a computer or printing equipment to output, store or process.
While the optical module 22 is moving reciprocally along the guiding track 23, it is also reading a correct picture 80, located on an upper side of the scanner body 20 to correct the color rank of the scanner, and perform positioning through selected patterns of the correct picture 80. However, due to space constraint on the upper side of the scanner, when the scanner document 70 has the selected patterns of the correct picture 80, positioning might fail.
Beside the scanner, copiers also use the correct picture 80 to perform correction of color, rank and positioning. Those types of plane image input apparatus all have the same problem.